package com.wc.alorithm_luogu.P1720;

import java.util.Scanner;

/**
 * @Author congge
 * @Date 2023/4/20 12:17
 * @description
 * https://www.luogu.com.cn/problem/P1720
 * 月落乌啼算钱（斐波那契数列）
 */
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        double gFive = Math.pow(5,0.5);

        double fN = (Math.pow((1+gFive)/2,n) - Math.pow((1-gFive)/2,n))/gFive;

        System.out.printf("%.2f",fN);
    }
}
